<style>
    /* (行内，id, 类，标签) */
    /* (0, 2, 0, 0) */
    #father #son {
        color: blue;
    }

    /* (0, 1, 1, 1) */
    #father p.c2 {
        color: black;
    }

    /* (0, 0, 2, 2) */
    div.c1 p.c2 {
        color: red;
    }

    /* 继承 */
    #father {
        color: green !important;
    }

    /* 继承 */
    div#father.c1 {
        color: yellow;
    }
</style>

<div id="father"
     class="c1">
    <p id="son"
       class="c2">白日依山尽，黄河入海流。</p>
</div>